1. Field of the Invention
The present invention relates to a hot water boiling apparatus of a storage type, using an electric heater as its heat source.
2. Description of the Related Art
Hot water boiling apparatuses using an electric heater as their heat source are classified into two types; an instant type and a storage type. The instant type is constructed so that water is instantaneously heated to a predetermined temperature by means of a large-capacity electric heater, to be supplied as hot water. The storage type is designed so that hot water of a predetermined temperature is previously stored in a hot water tank, and is supplied as required. Normally, the instant-type boiling apparatuses cannot produce hot water of a satisfactory temperature unless they use an electric heater with a large capacity of 5 to 20 kw. Therefore, the storage-type boiling apparatuses are exclusively put to household use.
Usually, the storage-type hot water boiling apparatuses comprise a hot water tank covered with a heat insulator. The lower portion of the inside of the tank is connected to a water supply pipe, while the upper portion is connected to a tap by means of a hot water supply pipe. A sheath-type electric heater is located in the lower portion of the hot water tank. The heater is supplied with electric power to heat all the water in the tank to, for example, 80.degree. C. during a time zone in which hot water need not be used, e.g., at midnight. There are two systems for supplying power to the heater; a full time power supply system and an off-peak power supply system. According to the full-time system, the power supply to the heater is started at any point of time, and is stopped when all the water in the tank attains a predetermined temperature. According to the late-night power supply system, the power supply to the heater is started at midnight during which the electric charges are relatively small, and is stopped when all the water in the tank attains a predetermined temperature. From the economical point of view, the late-night power supply system is used more widely. This system is provided with a timer switch which starts operation when the predetermined time is reached every day, for example, whereby the power supply to the electric heater is controlled. The timer switch is under the control of an electric power supplier, and is not accessible to users.
However, these hot water boiling apparatuses are subject to the following drawbacks, which will be described in connection with an apparatus using the all-time power supply system. When the electric heater is energized, the water in the hot water tank is gradually heated by a natural convection. At this time, the speed at which the water temperature rises depends on the capacities of the heater and the tank. It is hard, however, to incorporate a large-capacity electric heater in an apparatus for household use. In order to fulfill its function as a storage-type version, the apparatus must use a hot water tank with a capacity of at least several hundreds of liters. Accordingly, the speed at which the water temperature in the tank rises is not very great. When all the water in the tank is heated to 80.degree. C., for example, the power supply to the heater is stopped. Thus, in the hot water boiling apparatus using the all-time power supply system, it takes much time to heat the water in the tank to the proper temperature for use, i.e., 80.degree. C. Moreover, all the water in the hot water tank would be heated to the set temperature of 80.degree. C. without regard to the quantity of hot water actually required. Therefore, if the necessary quantity of hot water on the day and the capacity of the tank are 100 l and 300 l, respectively, energy will be wastefully consumed to heat 200 l of excessive water to 80.degree. C. This also applies to the case of the late-night power supply system. In a hot water boiling apparatus using the late-night power supply system, the power supply is allowed only during the limited time zone. To avoid a shortage of hot water supply in the daytime, therefore, the hot water tank must have a large capacity. Thus, the boiling apparatus requires a wider installation space, and may possibly waste electric power at a higher rate. In the conventional hot water boiling apparatus based on the late-night power supply system, moreover, hot water of 80.degree. C. is stored in the hot water tank at midnight. During the daytime, therefore, the hot water can be used at once as long as it is in the tank. In case of shortage, however, no hot water can be used in the daytime.